1. Field of the Invention
The invention relates to a measuring transformer for the potential-free measurement of currents or voltages, and to a static electricity meter including such a transformer.
2. Description of the Prior Art
FIG. 1 shows a known measuring transformer which comprises an annular magnetic core 1, a measuring conductor 2 for carrying the current I.sub.m to be measured, a pre-magnetizing winding 3 and an induction winding 4. The conductor 2 is passed through the closed magnetic circuit of the magnetic core 1, but could also be wound in a plurality of windings around the core 1, in the same manner as the pre-magnetizing winding 3.
In operation of this transformer, a pre-magnetizing current I.sub.v, which is preferably of a triangular waveform and which flows through the pre-magnetizing winding 3, produces a magnetic field which alternately drives the magnetic core 1 in both saturation directions, the magnetic core 1 operating as a magnetic field comparing means. If the current I.sub.m to be measured is of zero value, a symmetrical induction voltage U.sub.a is induced in the induction winding 4, and this induced voltage substantially comprises positive and negative pulses which occur at the moment of magnetism reversal of the magnetic core 1 and which follow at equal spacings in time. If, on the other hand, the instantaneous value of the current I.sub.m is greater than zero, then this current assists the magnetizing effect of the pre-magnetizing current I.sub.v, whereby there is a temporal displacement of the positive and negative pulses of the induction voltage U.sub.a. This temporal displacement can be evaluated as a measurement of the strength and direction of the current I.sub.m to be measured. The induction winding 4 is not absolutely necessary, as a voltage is also induced in the pre-magnetizing winding 3, and the variation of that voltage in time can be used in the same way as a measurement with respect to the current I.sub.m.
A known measuring transformer supplies an induction voltage U.sub.a whose pulses are relatively wide and have shallow flank angles of inclination. Moreover, the addition of the magnetic fluxes or magnetic fields which are involved, in the region of saturation of the magnetic core 1, is difficult to control, and this results in a complicated winding structure or costly compensation operations. In addition, the temporal displacement of the pulses relative to the passage through zero of the magnetic field is relatively great. This situation is altered only to a minor extent if the magnetic core 1 has a reduced portion, to reduce the saturation field strength. This known measuring transformer is therefore not suitable for the precision measurement of currents which vary rapidly.